Application research of inductive communication

    Abstract: Discuss the principles and characteristics of inductive communication, and give the basic structure of the equipment of the three-ring inductive communication system based on phase-locked loops; explain the application of inductive communication based on actual conditions.

    Keywords: Basic structure of induction communication phase-locked loop

In long and complex civil and military underground buildings, in various tunnels and mine underground passages, and in similar situations, experiments and theories have proven that it is difficult for radios from tens of kilohertz to hundreds of megahertz to propagate over hundreds of megahertz. meters distance. The propagation distance in the curved road is shorter, that is, there is no mobile communication in the above-mentioned underground situations similar to the ground [1]. But when there are metal conductors (wires) in the road, the situation will change greatly. Radio signals in certain bands can propagate over long distances (up to several kilometers). This radio signal propagates with the help of induction by conductors along the line. The communication accomplished is called inductive communication.

1 Principles and characteristics of inductive communication

Inductive communication is provided with insulated wires along the road (or a certain area); when the transmitter near the wire in the road (called the induction line) emits an electrical signal from the antenna, the signal from the transmitter antenna can be induced to the induction line, generating an induced current. This current generates signal field strength around the induction. The antenna receiver antenna along the way can receive the signal due to induction. After amplification and processing, the information sent by the transmitter is obtained and the communication process is completed. This is the principle of inductive communication. In inductive communication, the lower the operating frequency, the smaller the transmission loss and the greater the coupling loss; the higher the operating frequency, the greater the transmission loss and the smaller the coupling loss. The operating frequency that has actually been proven to be beneficial to inductive communication is 50kHz to 500kHz. The induction wire is generally an insulated wire with a diameter of 1mm to 2mm, 15cm to 25cm away from the wall, and set parallel to the road; the induction wire can be a single wire (one wire) to form a loop with the earth; it can also be a double wire to form a loop. Induction communication and leakage communication [2] are two mobile communication methods in underground tunnels and underground buildings. In comparison, induction communication has the advantages of simple structure, easy implementation, low investment, and quick results. Especially in emergency situations, it can be used in In unwired spaces, relying on other conductors as media, communication can also be completed when lines are interrupted; the main disadvantage of inductive communication is that the operating frequency is low, the capacity is small, and it is susceptible to interference.

2 Three phase locked loop induction communication system

This article uses phase-locked loop as the main body to form inductive communication equipment. The modulation, frequency multiplication and demodulation of the system all use phase-locked loop. Its working principle is shown in Figure 1. After the voice signal is filtered and audio amplified, the control end of the voltage controlled oscillator VCO1 is controlled, and the phase-locked frequency modulation is completed through the phase-locked loop composed of the phase detector PD1 and the low-pass filter LPF1. The FM signal is output by VCO1 and sent to PD2 for phase comparison with the VCO2 output signal. A phase-locked loop composed of PD2, LPF2, VCO2 and M frequency division circuit completes the M multiplication of the adjusted signal, doubling the offset frequency of the FM signal. The frequency multiplied signal is mixed with the crystal oscillator 2 signal, and the output difference frequency signal is filtered by LPF3, amplified by A2, frequency selected by LPF4, and sent to the antenna for transmission. The transmitted signal is a broadband FM signal with higher processing gain, thus improving the anti-noise performance of the system. When the system receives, the signal enters the LPF3 frequency selection through the antenna, is amplified by A2, and then mixed with the signal divided by LPF4 and crystal oscillator 1. After mixing, the signal is selected by BPF1 to select the difference frequency, and then amplified by A3 to enter the PD3 and VCD3 output signals. Phase comparison is performed; the phase-locked loop composed of PD3, LPF5, and VCO3 completes frequency identification; the demodulated signal is amplified by A4 and sent to the terminal (speaker) to complete the receiving process. Due to the use of phase-locked frequency discrimination, the noise threshold of the system can be lowered by 5dB ~ 10dB, further improving the system's anti-noise capability. This circuit uses phase-locked loop three times, which has good working stability, high frequency accuracy and strong noise immunity. Because the phase-locked loop circuit is simple and uses more low-pass filters and less band-pass filters, the circuit is further simplified, easy to implement, and has high operating reliability.

3 applications

(1) Inductive communication can be applied to mobile communications in environments such as mining underground tunnels and working faces. This kind of mobile communication is not a complete radio communication, but a semi-wired and semi-wireless mobile communication. Its system structure in the mine underground tunnel is shown in Figure 2. In the figure, it is assumed that the underground central square has tunnels extending in four directions. Each direction has a dedicated induction communication base station directly connected to the induction line, and the induction line extends into the tunnel. There are several mobile phones along the way, and the base station is connected to the switch to complete the transfer between mobile phone users; the switch has the function of connecting with the underground communication network.

(2) Inductive mobile communications can be applied to railway train wireless train dispatching systems. Here are examples. A special electric railway line is about 200 kilometers long; the average distance between adjacent stations is 10 kilometers. The area it crosses is an arid desert area. There is no support for public network mobile communications. A dedicated train dispatching wireless system is required. The induction method is adopted after demonstration. The system schematic diagram is shown in Figure 3. Among them, S1, S2, S3... are induction wireless base stations at stations along the line, A is the antenna, and D is the overhead power line of the railway train. Antenna A is a linear antenna (25 to 30 meters long), which is set up parallel to the power line near the station; BS is an on-board station on the train, and its antenna BA is set up parallel to the power line at the top station of the locomotive; the mobile on-board station BS is located at the locomotive driving station room and the commander's room; each base station is connected to the main train call line switch C5 of the entire line through wired transmission lines, and C5 is linked to the public network. The main train dispatching room can communicate with the train commanders and drivers' on-board stations along the route via wired transmission through each base station; the train can communicate with the station and the main dispatching room when the train is running in plains, mountains, or bridges and tunnels. Each base station signal can cover part of its own station and adjacent stations. The system structure is simple, easy to implement, requires less investment, has quick results, and works reliably and effectively.

(3) Teaching broadcast system. The school needs a broadcasting and listening system for English mathematics for listening teaching training and consideration. Induction transmission lines can be set up in the teaching building area or corridors. The transmitter output can be directly connected to the induction line. The terminals of the induction line are connected to matching resistors along the line. Students in surrounding classrooms can listen using simple wireless receivers. This is a one-way broadcast system. The signal only acts in a limited area and operates in the frequency band below the intermediate frequency. It has no interference with other systems. It has a simple structure, low investment and is easy to implement.